Photodynamic therapy (PDT) shows promise as a treatment of cancer. PDT, first used in 1975, is based on the concept that light irradiation can change an inert substance into an active one (1). In PDT, a specific light-sensitive agent, the so-called photosensitizer, is administered systemically to a cancer patient. Light of a specific wavelength is delivered to the tumor and activates the photosensitizer. The activated molecule transfers an electron to an adjacent oxygen molecule and generates oxygen radicals, or the energy is transferred from the activated photosensitive molecule to an oxygen molecule, generating an excited singlet oxygen molecule. These reactive oxygen species have very short lifetimes, but are extremely reactive and usually induce a cytotoxic reaction or cell destruction, respectively.
There have been several studies published describing the use of PDT to treat cancer in both animals and humans, including the treatment of lung and brain cancers. However, one main limitation of the photosensitizers used is that they absorb light at a relatively short wavelength (typically 600-700 nm), meaning that light cannot penetrate deep into the tissue (generally up to 1 cm). A commonly used clinical photosensitizer is Photofrin porfimer sodium, which has the side effect of causing prolonged skin photosensitivity that results in patients having to be protected from sunlight for several weeks. Despite these limitations, PDT has now achieved the status of a standard treatment modality for centrally located early-stage lung cancer.
A less invasive type of PDT is performed with a bronchoscope for the treatment of bronchopulmonary malignant neoplasia (2). In this therapy, the endobronchial tumor is presensitized by administration of the sensitizing photochemical. After a time interval, bronchoscopic illumination (exposure to laser light) is performed to achieve cancer necrosis. PDT is now indicated in both early and advanced stage cancers of this type.
Another application of PDT is done in combination with surgery. For example, in a phase II trial with 22 patients with non-small-cell lung cancer (NSCLC) with pleural spread, the patients received the photosensitizer porfirmer sodium 24 hours before surgery, at which time all the gross tumor was resected and followed by illumination of the hemithorax with 603 nm light (3). The median survival was 21 months, which was viewed as encouraging and warranting further evaluation of this therapy.
The availability of alternative methods of using photodynamic therapy to treat cancers is desirable.